This invention concerns a method for the measurement of the equivalent damping resistance of a LC resonant circuit and its application to devices which measure the distance to an electrically conductive object based on changing losses in the LC-resonant circuit with varying measuring distance, specifically proximity switches.
Numerous technical measuring methods are based on the determination of the damping of LC resonant circuits. The precise measurement and control of the equivalent damping resistance of this sensing circuit type is therefore a central problem of many electronic devices. In the following a device for the determination of the distance relative to an electrically conductive object (target) on the basis of the damping of a LC resonant circuit is used for the illustration of the new method and as an example for a circuit implementation of the present invention. Devices based on the damping of a LC resonator are widely used in industry, specifically as proximity switches.
The accuracy with respect to the measuring or switching distance, resp. of such devices is determined to a great extent by the circuit principle utilized for the excitation of LC resonant circuit. Generally a distance dependent direct current or oscillation voltage value is formed and, in the case of a proximity switch, is used for controlling a switch. In doing so, a principal problem is the temperature-constancy of the switching point and its dependence on the tolerances of the circuit components utilized and of the housing installation, which calls for a compensation after the completion of the manufacturing.
For reasons of cost, the choice among the many and diverse known oscillator structures and the possibility of compensation after the manufacturing (e.g., by means of laser trimming) is limited.
In general, the core of an oscillating circuit with a LC resonant circuit is formed by a feedback amplifier. If the feedback is xe2x80x9cin phasexe2x80x9d and if the circuit amplification is greater than one, then the circuit oscillates with a frequency defined by the LC resonant circuit. The amplitude of the oscillation generated in the case of simple oscillators, apart from a dependence on the LC resonant circuit damping, is above all defined by a so-called non-linearity, e.g., inasmuch as the limit of the dynamic control range is reached, which renders a further increase of the oscillation voltage impossible. By this saturation level dependent on the circuit damping, the working point currents of the circuit are changed; this change can be utilized as a measure for the oscillating circuit damping and therefore as a measure for the distance.
As an alternative to this method, the controlling of an oscillation amplitude to a fixed value by means of a linear controlling of the amplification factor of the excitation amplifier presents itself, wherein the control value is a measure for the damping of the oscillating circuit.
Types of circuit of this kind, however, are only to a very limited extent suitable for fulfilling the combined characteristics listed above, for the following reasons:
The derivation of the distance information from a more or less undefined mixture of oscillating circuit damping and oscillating range saturation, as takes place in simple oscillators, because it can only be theoretically described with difficulty, is not suitable for highly accurate and digitally trimmable oscillators;
The method with a controlled oscillation amplitude manifests a too great inertia (in the case of moving measuring heads). Equally the exploitation of the commencement of the oscillation as a proximity switching criterion has to be eliminated for reasons of speed.
The invention therefore sets itself the objective of creating a circuit structure, with which the requirements mentioned above are fulfilled and which overcomes the disadvantages of the alternative methods.
The improved method in accordance with the invention is based on a novel oscillator concept, which provides the possibility for fulfilling in combination the following characteristics:
Basic excitation circuit of the LC resonant circuit which generates an oscillation voltage proportional to the damping resistance of the LC resonant circuit and which is controllable by the exiting circuit in a well-defined manner.
Compensation of the temperature-dependence of the LC-resonant circuit adjustable independent of the basic circuit.
xe2x80x9cElectronicxe2x80x9d compensation of the manufacturing tolerances after completion of the manufacture.
2-terminal oscillator (no middle tappings on the coil).
Monochip-integration of all the functions.
The present invention is based on an oscillation circuit which uses the LC resonant circuit in combination with an excitation circuit whose performance is controlled by one single resistor connected to the exciting circuit. This feature makes it suitable for monochip integration. Using this oscillation circuit in connection with an inductive sensor probe for proximity switches, the switching distance is adjusted by the distinguished resistor mentioned above. According to special embodiments, additional means are provided for the temperature compensation and xe2x80x9celectronicxe2x80x9d trimming of the device.
The requirement for a 2-terminal coil without a middle tapping can be fulfilled by a two-stage excitation amplifier. Circuits of this kind have been suggested in the literature. These circuits, however, only have a limited dynamic controllability and apart from this are not suitable for CMOS technology. In order to circumvent this disadvantage, one can in accordance with the invention extend circuit types of this kind, as is explained below in detail in connection with the discussion of the Figures.